The present invention relates to a diamond coated button insert for use in drilling, the insert having a cutting surface which is of symmetrical configuration about a center axis of the insert.
Depicted in FIG. 1 is a conventional drill bit 10 and hammer 12 used in a down-the-hole percussive (DTH) rock drilling method. The drill bit is alternately rotated and impacted by the hammer. The impact provides percussive energy for breaking rock. A top-hammer method is similar to the down-the-hole method, except that the impact is applied at the ground surface rather than by means of a down-the-hole hammer.
In each method the drill bits are usually equipped with buttons 14 which make contact with the rock. The buttons are made of a wear resistant material such as cemented carbide. Although cemented carbide is very wear resistant the buttons tend to wear out much more quickly than is desirable. This is particularly a problem with the gauge buttons 14A which are most exposed. For this reason diamond enhanced cemented carbide buttons are sometimes used, as illustrated in FIG. 2 wherein a diamond layer 16 is applied to the top of the carbide, the diamond layer thus defining a spherical dome surface 16A having a radius of curvature r. The dome surface 16A is of symmetrical configuration with respect to a center axis A of the button.
Diamond has superior wear resistance, but is unfortunately very brittle. One area of weakness is along the outer periphery 16P of the dome. Here the diamond layer is thin and the material has weak points in the diamond and cemented carbide interface. If the outer periphery is in contact with the rock during drilling, the diamond coating has a tendency to spall off. Sometimes a crack forms which causes the entire button to break. To reduce that problem, the dome can be defined by a relatively small radius of curvature, whereby the outer periphery 16P of the dome will be out of contact with the rock, as demonstrated by the button 14' shown in FIG. 3.
However, reducing the radius serves to increase the rate of wear at the crown 16C of the diamond, and increase the susceptibility of the crown to the formation of cracks that can cause the entire button to break. Those shortcomings can be minimized by increasing the dome radius, but then the outer periphery of the dome will contact the rock during cutting, as demonstrated by the button 14" shown in FIG. 4, whereby the tendency for spalling to occur increases, as noted earlier.
It will be appreciated, then, that selecting an "optimum" dome radius has involved a compromise. Typically, the radius of the diamond dome has been selected to be one hundred ten percent (110%) of the radius D of the cylindrical shank 18 of the carbide base.
In lieu of constant-radius domes, it has been proposed to form the crown of a smaller radius than the outer periphery. The drilling rate is thus increased, but so is the wear rate.
Therefore, it would be desirable to provide a button insert of the type having a diamond layer which forms a dome-shaped cutting surface configured symmetrically about a longitudinal axis of the insert, which is highly resistant to spalling and crack formation, and yet exhibits a relatively long life.